This invention relates to an apparatus for tracking phase discontinuities introduced into an incoming signal at known instants, particularly phase discontinuities caused by switching between antenna elements in a navigation system providing orientation information.
Current radio navigation systems are mainly land-based and suffer from system inaccuracies or poor surface coverage. One such system in commercial use is Loran C, which is capable of giving positional information to an accuracy in the order of 0.25 nautical miles. The range of Loran C is limited to coastal areas, and it is not feasible for the system to provide world-wide coverage. OMEGA is a system presently consisting of eight stations spanning the world and transmitting unique identification data on the VLF Band. The accuracy of OMEGA is severely limited by propagation errors, and an accuracy in the order of 2 to 4 nautical miles is all that can be expected.
It has long been recognized that satellites can be used to provide a world-wide accurate radio navigation system. The major existing satellite system in use is called TRANSIT. This system employs six satellites in low polar orbits. The TRANSIT system makes use of doppler shifts to derive velocity vectors from which positional information can be derived. The main disadvantage of the TRANSIT system is that the coverage is periodic, with the interval between possible fixes being as much as ninety minutes.
In order to overcome the disadvantages of the TRANSIT system, which will be gradually phased out, work is now in progress to install a brand new satellite navigation system known as NAVSTAR. NAVSTAR will eventually consist of a constellation of eighteen satellites in six 12-hour orbits with three active spares. The disposition of the orbits and the satellites within the orbits will be such that at least four satellites will be within view of any location on Earth at any particular time. Each satellite transmits two link signals, L1 and L2, at 1,575.42 MHz and 1,227.6 MHz, respectively. The in-phase component of L1 is modulated with a precision (P) code and the quadrature component of L1 is modulated with a clear acquisition (C/A) code. L2 is modulated only with the P code. The C/A and P codes enable an authorized user to lock onto the signals and extract satellite identification and timing information. In addition to the codes, the L1 and L2 signals are also modulated with data at a lower bit rate giving precise ephemeris information pertaining to the satellite orbit. The authorized user can calculate from this information the position of the satellite and hence, by taking a fix on a plurality of satellites simultaneously, can calculate his exact coordinates. By measuring the doppler shifts in the received signals, the user can also derive velocity information.
The basic NAVSTAR system will not, however, give information about the orientation of the user, namely heading and attitude information. Clearly the heading of a vehicle, such as a ship or aircraft, may be quite different from the direction of its velocity vector. This information can in theory be derived if at least three spaced antenna elements are used, and interferometric measurements are made to determine the orientation of the base line vector separating the antenna elements relative to the transmitting satellite. By observing a number of satellites, the user can derive complete information pertaining to the orientation of the platform mounting the antenna. A significant problem therefore arises as to how to measure accurately the phase difference between the antenna elements.